It's a good idea to keep the scope tube as low as possible to the bore, to reduce off-set. With a 40mm objective, that usually means medium rings.
I put a Leupold 3-9x40 with low rings on a Marlin .17 HMR, and there is just barely enough room between the scope's objective bell and the barrel to clean the surfaces. I'm planning on replacing them with mediums.
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